1. Field of the Invention
The present invention relates to a refrigerating and heating apparatus using a solid sorbent.
Therefore the invention relates to heat pumps and refrigerators using for their operation fixed beds of solids interacting with a fluid, a gas or a gas mixture. The solids adsorb or desorb the gas as a result of a temperature variation in said solids.
2. Discussion of the Background
Refrigerating and heating apparatuses using intermittent cycles having one adsorber are known. These known apparatuses suffer from the two disadvantages of low efficiency (essentially due to the sensible heat involved during wide temperature variations of the adsorber), the coefficient of performance (COP) relative to the cold production being below 0.5, and the fact that they also produce heat or cold in a discontinuous manner.
In order to obviate these two disadvantages, apparatuses using regeneration cycles with several adsorbers have been produced.
The basic idea of these apparatuses is to use the sensible heat and the latent adsorption heat given off by an adsorber during its cooling and during adsorption in order to preheat another adsorber during heating and desorption.
The recovery of internal heat on one adsorber for preheating the other adsorber leads to economies with regard to the heat taken from the hot source of the apparatus and therefore leads to an improvement in its efficiency.
In order to achieve this, use has been made up to now of two regeneration types, corresponding to two types of "managing" the heat in the adsorbers.
In a first type use is made of uniform temperature adsorbers. A heat transfer fluid circulates in a heat exchanger placed in each of the adsorbers and said exchanger and the operating conditions are such that the lowest possible temperature gradients exist within each adsorber and between the corresponding solid adsorbent bed and the heat transfer fluid.
When operating the corresponding apparatus, there is a heat recovery phase during which the heat transfer fluid flows between the two adsorbers, one of them being initially at a high temperature, whereas the other is initially at a low temperature. During this phase, no heat is taken from an external heat source. This phase is interrupted when the two adsorbers are at the same temperature.
For further details reference should be made to documents (1) to (3) which, like the other documents referred to hereinafter, appear at the end of the present description.
It has also been proposed to use more than two adsorbers (cf. particularly document (3)) for "managing" the heat. In the case of such apparatuses, very interesting experimental results have been obtained, a cold production COP of 1.06 having been obtained with a cycle with three adsorbers.
A second control or management type consists of creating high axial temperature gradients within the adsorbers. It is then theoretically possible to cool an adsorber by a cold heat transfer fluid and obtain a hot transfer fluid at the exit from said adsorber as a result of high axial temperature gradients within said adsorber. This hot fluid is then used for regenerating a second adsorber.
This procedure is described in greater detail in documents (4) and (5).
Thus, this procedure uses two adsorbers, which are connected in series and two heat exchangers (one serving as the hot source and the other as the cold source), and which are intercalated between these two adsorbers, the heat transfer fluid flow direction being reversed at the end of an operating half-cycle.
Such a procedure is described in detail in document (6).
It has been theoretically demonstrated that, if steep temperature fronts are obtained, regeneration can be very efficient and cold production COP levels exceeding 1 or more or even reaching 2 could be obtained.
It has even been proposed to use regeneration cycles using temperature fronts in four reactors, which would make it possible to obtain COP's above 2 (cf. document (7)).
However, up to now all the experimental attempts made with this heat front process have only led to very inadequate results, because the highest cold production COP obtained is only 0.75 (cf. document (8)), which is inferior to the results obtained with cycles using three uniform temperature adsorbers.
The main reason for these poor results if the difficulty in obtaining the propagation of the steep temperature fronts. This is mainly due to the materials used as adsorbents. They are constituted by fixed beds of granular adsorbents having poor heat transfer characteristics, namely thermal conductivities of approximately 0.1 to 0.3 W.m.sup.-1..degree.C..sup.-1 for said adsorbents and exchange coefficients on the wall below 200 W.m.sup.-2..degree.C..sup.-1 such conditions being very unfavorable for obtaining steep temperature fronts.